Field of the Invention
This invention relates to an accessory device for a quench valve of a cryostat, in particular for use in a magnetic resonance imaging (MRI) system. Furthermore, this invention relates to a method of enabling a cryostat containing a cryogen to be safely transported by air transportation.
Description of the Prior Art
Superconducting magnet systems are used for medical diagnosis, for example in magnetic resonance imaging systems. A requirement of an MRI magnet is that it produces a stable, homogeneous, magnetic field. In order to achieve the required stability, it is common to use a superconducting magnet system which operates at very low temperature. The temperature is typically maintained by cooling the superconductor by immersion in a low temperature cryogenic fluid, also known as a cryogen, such as liquid helium.
The superconducting magnet system typically comprises a set of superconductor windings for producing a magnetic field, the windings being immersed in a cryogenic fluid to keep the windings at a superconducting temperature, the superconductor windings and the cryogen being contained within a cryogen vessel.
Superconducting magnets are susceptible to quench events, in which, for one of a number of reasons, part of the superconducting magnet ceases to be superconducting. The resulting resistance in part of the magnet causes heat due to the current flowing through it. This rapidly causes further parts of the superconducting magnet to cease superconducting. The result is that all of the energy which was stored in the magnetic field of the magnet is suddenly released as heat. In a superconducting magnet cooled by a liquid cryogen, this typically results in rapid boil-off of a large volume of the cryogen, with gaseous and liquid cryogen being expelled from the cryostat at high speed. During a quench, it is essential that the escaping cryogen gas is allowed to exit the cryostat in a safe manner. The exit point typically opens by responding to an increase in the pressure within the cryostat. It is known to provide a quench valve to control the exit point. The quench valve is closed until a certain pressure is reached within the cryostat. Once the cryostat pressure reaches the certain value, the quench valve is opened by the pressure acting upon it.
During transportation of an already assembled system, filled with cryogen, no cooling can be provided to the cryogen, which leads to a heat input into the cryostat, leading to a boil-off of cryogen. Therefore, during air transportation, relief devices must be available in order to guarantee a pressure-relief to protect against overpressure. In other words, a significant build-up of pressure within the cryostat shall be prevented.
However, the change of atmospheric pressure during an air shipment, even in a pressurized compartment, can cause a problem with the relief devices employed. Ordinary relief valves can freeze and plug up following rapid ejection of cold gas following altitude changes. For this reason, for air transportation, each magnet system has to be fitted with an absolute pressure relief valve, which is unaffected by atmospheric pressure. In addition, in order to comply with safety regulations, an independent second device has to be present, which second device can be a gauge device.
It is permissible to use the existing quench valve as the gauge device. However, the differential pressure required to crack the quench valve is less than the differential between the pressure within the magnet system and the pressure within the hold of the air craft during air transportation. Therefore, the quench valve would lift and vent excessive cryogen gas. In order to overcome this, it is known to blank off the outlet of the quench valve by an air tight plate fitted with e.g. a 13 PSIG valve. Additionally, a hand valve is fitted, which may also be used to relief pressure before removing the plate. The whole assembly needs to be leak tight and fully tested, making this an expensive solution. Furthermore, the assembly is discarded after arrival on operational site.